For cutting human or animal hard tissue, such as bone or nail tissue, various methods and devices are applied today. In recent years, photoablation using laser beams turned out to be a feasible alternative to known tools and methods. Particularly, in osteotomy, laser induced photoablation became of increasing interest as an alternative to known mechanical tools such as saws, chisels, or drills. The aim of such laser induced photoablation is to increase preciseness and operability while decreasing collateral damage, e.g., caused by direct contact of the mechanical tools with the bone.
For example, in WO 2011/035792 A1 a computer assisted and robot guided laser osteotome medical device is described. This medical device includes a laser head mounted to a robotic arm. The robotic arm has several degrees of freedom such that the laser head can be precisely adjusted in preferred positions and orientations. Like this, it is possible to precisely provide a laser beam onto bone tissue and to photoablate the bone such that it is cut along a predefined osteotomic line. For determining and adjusting the beam position in relation to the bone, the medical device has an autotracking system. Using the autotracking system, the position and orientation of the bone can be monitored and the laser head can be adjusted in order to prevent a deviation of the cutting from the predefined osteotomic line.
A common problem in known laser induced photoablation of human or animal hard tissue relates to controlling cutting depth and beam intensity. In contrast to laser induced photoablation widely used in micromachining of non-biological materials such as metals and plastics, issues with respect to collateral damage are of crucial importance when photoablating human or animal hard tissue. Such collateral damage, e.g., carbonization, can occur due to heating caused by inappropriate laser beam intensities in tissue neighboring the osteotomic or cutting line. Or, collateral damage can also occur due to photoablation beyond the depth of the targeted hard tissue. Making these problems even more difficult to handle, in contrast to the mentioned non-biological materials, human or animal hard tissues of the same type usually differ from one individual to the other. Furthermore, human or animal hard tissues usually are not homogeneous such that the photoablation properties of the tissue can vary within one single tissue target, particularly, depending on the cutting depth. For preventing such excess or unwanted photoablation, depth of the photoablation in the tissue is usually optically monitored, e.g., using optical coherence tomography (OCT). However, such monitoring is, on one hand, usually rather complicated and can, on the other hand, be impaired by other factors of the photoablation such as by debris, water, or blood.
Therefore, there is a need for a method and device allowing convenient improved photoablation of human or animal tissue using a laser beam particularly in terms of collateral damage caused to the tissue by the laser beam.